Synchronizing system



1949 H. B. FLEMING ETAL 2,491,804

SYNCHRONIZ ING SYSTEM Filed Nov. 29, 1946 2 Shets-Sheet 1 Pi .I. 1 3

TELEVISION 6 RECE'VER LsvucHRomzme sYNcHRomzme VERTICAL SIGNAL PULSEDEFLECTION CLIPPER SEPARATOR '1 GENERATOR I5 HORIZONTAL DEFLECTIONGENERATOR 35 1 H t2; LsAwTom H I: -1. l0 WAVE GENERATOR 22 2 Inventors:H ugh B.FIeming, George M.BTOWT|,

Their Attorney.

2 1949 H. B. FLEMING ET AL 2,491,804

SYNCHRONIZING SYSTEM Filed Nov. 29, 1946 2 Sheets-Sheet 2 F i2 3. l5 r'TO HORIZONTAL DEFLECTION GENERATOR SAWTOOTH a WAVE R GENERATOR l 34 55FROM 9 SYNCHRONIZING l PULSE SEPARATDR Fig.4.

Inventors: Hugh 13. Fleming, George M. Bvown,

g m DM Their Attorney.

Pat ented Dec. 20, 1949 UNITED STATES PATENT OFFICE SYNCHRONIZING SYSTEMApplication November 29, 1946, Serial No. 712,998

Claims. 1

Our invention relates to synchronization of a source of saw tooth waveenergy with periodic energy pulses such as those from a televisiontransmitter.

In present-day cathode ray television systems, periodic high energypulses are transmitted as successive scanning cycles are commenced.These pulses are then used at the receiver to trigger the sweep circuitsof the cathode ray tube used for image presentation, thereby causing theposition of the ray to correspond with scanning operations at thetelevision transmitter and the image on the tube screen to reproduce thevisual program.

It is an object of our invention to provide improved means tosynchronize a sawtooth wave generator in response to periodic energypulses.

It is further an object of our invention to provide synchronization of awave generator in response to applied energy pulses in a manner whichprevents false operation from random noise pulses.

Still another object of our invention is to synchronize the sweepcircuits of a cathode ray tube in a manner that permits a plurality ofsweep cycles for each synchronizing pulse while at the same timeproviding uniform sweep cycles and preventing random noise pulses fromtriggering the sweep circuits.

A further object of our invention is to synchronize a sawtooth wavegenerator in a manner permitting it to be rapidly synchronized after itis started and at the same time to reduce to a minimum the possibilityof triggering by random noise pulses.

Briefly, our invention resides in stabilizing the frequency of amultivibrator by two separate control signals: (1) a received controlsignal, which may for example be derived directly from a televisionreceiver output, and (2) a control signal derived from a phase sensitivesystem. The former signal is fed directly to the multivibrator in amanner tending to cause it to change from one operating condition to theother, and the multivibrator voltage is utilized to trigger a sawtoothsweep generator each time a synchronizing pulse is received. The lattersignal, which has a magnitude dependent on the phase relationshipbetween the sawtooth sweep wave and the applied synchronizing pulses, isalso applied to the multivibrator in a manner to vary the naturalfrequency at which that unit changes from one operating condition to theother. Under starting conditions, the control signal derived directlyfrom the receiver output causes the multivibrator to come intosynchronism rapidly while under steady state conditions the controlsignal from the phase sensitive system maintains the frequency and phaseof the multivibrator at the correct value despite random noise pulsesthat may be present in the receiver output. The sawtooth generatoroutput may then be used to cause synchronized scanning of the cathoderay tube beam, as is well known to the art.

The features of our invention which we believe to be novel are set forthwith particularity in the appended claims. Our invention itself,however, together with further objects and advantages thereof, may bestbe understood by reference to the following description taken inconnection with the accompanying drawings in which Fig. 1 is a schematicdiagram of a television receiving system incorporating synchronizingcircuits employing the principles of our invention; Fig. 2 shows Waveforms illustrating the operation of the circuit of Fig. 1; Fig. 3 is aschematic diagram illustrating a second embodiment of our invention,particularly suitable for use where a plurality of sweep cycles aredesired between successive synchronizing pulses; and Fig. 4 shows waveforms illustrating the operation of the circuit of Fig. 3.

To aid in understanding our improved synchronizing system it is shown inFig. 1 in its application to a television receiving system, theconventional portions of which are shown only in simplified schematicform, since their details form no part of the invention. Thus,television carrier signals received by antenna l are supplied -totelevision receiver 2 which may conventionally comprise the highfrequency amplifier, oscillator, intermediate frequency amplifier,demodulator and video amplifier circuits of a wide-band superheterodynereceiver. The detected video signals are supplied over conductor 3 inwell-known manner to the intensity control element of a cathode ray tube4 and also to the usual synchronizing signal clipper 5 and synchronizingpulse separator 6 from which the horizontal and vertical synchronizingpulses are derived. These pulses are supplied to synchronize thehorizontal and vertical deflection generators l and 8 respectively, theoutputs of which are applied to the corresponding deflecting elements oftube 3 to produce the picture scanning action as is wellunderstood inthe art.

Apparatus embodying our invention, now to be described, is showndiagrammaticall in Fig. 1 between the separator B and the deflectiongenerator l in the horizontal synchronizing circuits. The horizontalsynchronizing pulses from separator 6 are coupled through capacitor 9 toa multivibrator I0. This multivibrator may be any one of the well-knowntypes having a free-runhing or natural frequency of oscillation andcapable of being triggered by applied synchronizing pulses. Theparticular circuit shown in Fig. 1 utilizes the two triode sections 1 laand Nb of electron discharge device II. The anodes of device H areconnected through resistors l2 and I3 respectively and conductor M to asuitable source of anode potential, indicated conventionally by B-{. Thecontrol grids are connected to a suitable source of bias potential,derived from a potentiometer l6 across the anode supply, throughresistors I 8 and I9 respectively. The potentiometer I6 is bypassed forthe operating frequency of multivibrator ID by a capacitor IT. Thecontrol grid of each tube section I la and I lb is cross-connected withthe anode of the opposite section through capacitors 20 and 2|respectively. The grids are also connected to ground through resistors22 and 23 and a common capacitor 2 3. In addition to the direct currentpaths to ground from the grids through resistors l8 and I9 and biaspotentiometer l5, paths are also provided through resistors 22 and 23and thence through a common conductor 25 and the anode space path of adischarge device 26, for a purpose presently to be described.

In the multivibrator I0, capacitors 20 and 2| are alternatively chargedthrough anode resistors 18 and I9 and discharged through the gridnetwork just described. As is well-understood in the art, multivibratorl successively passes between two conditions of operation, onecorresponding to conduction through first section Ila and the othercorresponding to conduction through second section llb. Hence,approximately square- Wave voltages are developed across the anoderesistors I2 and 13. The natural, or free-running, frequency ofoperation of multivibrator H] can be adjusted by the bias potentiometer[6. It is also dependent upon the resistance in th paths from the gridsthrough discharge device 26. Device 23 therefore exercises a controlaction on the multivibrator frequency, as will become more fullyapparent as the description proceeds.

Square wave potentials appearing at the anode of section llb ofmultivibrator H] are supplied through capacitor 32 to synchronize theoperation of a sawtooth wave generator 33. Since many suitable circuitsfor this purpose are wellknown to the art, generator 33 has beenindicated Only in block form to simplify the drawing.

The sawtooth waves from generator 33 are supplied over conductor 34 to abalanced peak detector 35 shown as comprising two duo-diodes 33 and 3?connected in a bridge network. Across one diagonal of the bridge,between points a and b, are connected the sawtooth wave generator outputin series with an output impedance comprising potentiometer 38 andcapacitor 39 in parallel. Across the other diagonal of the bridge,between points 0 and d, is connected an impedance network comprising aresistor All and capacitor M in parallel, both in series with thesecondary winding 42b of a transformer 42. P0- tentials appearing onpotentiometer 38 are supplied to the control grid of discharge device 26through adjustable tap 43.

Synchronizing pulses from separator 6 are also supplied over conductor50 to the control grid of a discharge device connected in a cathodefollower circuit. Pulses appearing across the out put resistor 52 aresupplied to the primary winding 32a of transformer 42 through a couplingcapacitor 53 for a purpose now to be described.

The operation of the balanced peak detector 35 is known to the art andhas been described in detail in an articl beginning on page 7 of theProceedings of the I. R. E., January 1943, to which reference may bemade. Briefly, the four diode elements are so poled that current mayflow in either direction between the input and output circuits (i. e.between points a and b) when they are in conducting condition. Thesawtooth waves applied to point a from conductor 34 therefore tend toproduce equal symmetrical waves in the output and no resultant directcurrent component at point b. However, auxiliary pulses, correspondingin frequency and phase to the synchronizing pulses, are impressed fromtransformer secondary 42b across the opposite diagonal cd of the diodebridge in push-pull relation. Current flow due to these pulses is insuch direction as to build up a unidirectional potential across resistor40 and capacitor 4| tending to bias the diodes to nonconductingcondition. The Values of resistor 40 and capacitor 4| are chosen toeffect peak rectification of the applied pulses, preventing conductionthrough the diodes and. between points a and 1) except when thesynchronizing pulses occur. During the synchronizing interval, capacitor39 is charged in a sense depending upon the relative phase between thesawtooth and synchronizing waves so as to hold point I) at a directpotential level nearly equal to that of point 0. The potentials appliedfrom transformer secondary 421) do not appear across capacitor 39because the charge built up across capacitor 3| substantially cancelsthem from the standpoint of current flow between points a and b\ Thebalanced peak rectifier 35 may be regarded as a means to develop aunidirectional bias potential at point D dependent upon the phaserelationship between the synchronizing pulses and the sawtooth wave. Theoperation of the circuit of Fig. 1 will be better understood byreference to the wave shapes shown in Fig. 2 on a common time scale.Curve A represents the horizontal synchronizing pulses supplied from theoutput of pulse separator 6. Curve B represents the corresponding outputwave from multivibrator I0 when perfect frequency and phasesynchronization exist. Curve C illustrates the shape and phase of thesawtooth wave supplied to point a from generator 33 under suchconditions. It will be observed that the retrace portion 44 of curve Cis midway between its maximum and minimum values at the time the leadingedges of the synchronizing and multivibrator pulses occur. Inasmuch asit is this voltage that determines the voltage of point b, that pointhas a corresponding voltage under these conditions. If, however, theoperation of multivibrator I0 is delayed with respect to thesynchronizing pulses of curve A, each positive portion of wave B willoccur at a later instant of time than is shown in curve B. Curve Dillustrates such a condition. In this case the output wave fromgenerator 33 is also shifted slightly in phase as shown in curve E, thisvoltage now having peak magnitude at the instant each pulse of wave A isapplied. Hence, the potential at point I) is greater than in thecondition or normal phase synchronism. Should the operation ofmultivibrator 10 lead in phase oped across them is substantiallyconstant between successive synchronizing pulses.

A selectable portion of the control potential acrossresistor 43 issupplied to the control grid of discharge device 26. As shown in Fig. 1,device 26 is connected as a direct current amplifier to provide acontrollable resistance between point e and ground. As more positivebias is applied to the grid of amplifier 26, its effective resistancebetween point e and ground is decreased. This efiectively decreases thetime constant of the discharge network for capacitors 20 and 2| inmultivibrator l0, increasing its free-running frequency and therebycounteracting the lagging phase condition which produced the increasedvoltage across capacitor 39. Reverse action takes place upon reductionin bias due to a leading phase condition. Hence, the sawtooth wavegenerator 33, the balanced detector 35 and the direct-current amplifier26 comprise a closed, phase-control system tending to maintainsynchronism between multivibrator l and the applied synchronizingpulses.

The pulses from multivibrator I0 are supplied over conductor [5 tosynchronize the horizontal deflection generator I in well-known manner.

It will be observed that the phase-control system just described couldbe utilized to effect horizontal synchronization if capacitor 9 wereomitted and synchronizing pulses were supplied over conductor 50 to thephase-control circuit alone. However, the synchronizing action of thephase-control circuits alone is relatively Weak when the system is farfrom synchronism, for successive synchronizing pulses appear at greatlydifierent points on the sawtooth wave. Ordinarily, this would render theentire system very slow in achieving synchronism on starting, therebycausing an undesirable delay while the oscillator is slowly pulled intostep. In accordance with our invention this difficulty is avoided by theuse of a supplementary synchronizing system which gains control duringthe starting period but is not as efiective as the phase controlcircuits under normal operation. In the circuit shown in Fig. 1, weachieve this supplementary synchronization by the use of the couplingcondenser 9 which also applies a small direct synchronizing pulse tomultivibrator l0. Inasmuch as this pulse exerts a substantialsynchronizing effect even when the unit is first started, normaloperation is quickly obtained and the delay otherwise associated withthe phase-control network is avoided. Since the direct synchronizingvoltage applied through condenser 9 is made relatively small, randomnoise voltages from separator 6 do not cause multivibrator l0 to make afalse change. Hence, freedom from nois triggering, which ischaracteristic of the slow acting phase synchronizing system, isobtained without excessive synchronizing time requirements.

An alternate embodiment of our invention is shown in Fig. 3, in whichelements corresponding to those to Fig. l have been given correspondingreference numerals. The conventional portions of the televisionreceiver, shown in block form in Fig. 1, have been omitted from Fig. 3and only the features essential to a clear understanding of this form ofsynchronizing system have been shown. This embodiment differs from thatof Fig. 1 in that the multivibrator I0 is designed to operate at afrequency which is a multiple of the synchronizing pulse frequency, anda gear multivibrator 60 is added. The multivibrator 60 is conventional.It is similar to multivibrator I0 in circuits and operation but isnotprovided with the frequency controls. Its circuit constants areadjusted to make its natural frequency of operation nearly equal to thefrequency of the synchronizing pulses from separator 6. Pulses frommultivibrator I0 are supplied through capacitor 6| to causemultivibrator 6D to look into step with multivibrator I0.

Figure 4 shows wave shapes on a common time scale illustrating theoperation of the circuit shown in Fig. 3. Curve A, Fig. 4, shows thesynchronizing pulses from pulse separator 6, as before, and curve Bshows the voltage wave developed by multivibrator I0. In this case,multivibrator lll is shown as making two cycles for each synchronizingpulse. Other ratios may of course be used. Curve C shows the output ofmultivibrator 60, this voltage corresponding in frequency to the appliedsynchronizing pulses. Curve D illustrates the sawtooth wave output fromgenerator 33 under conditions of phase balance. Inasmuch as the latterwave corresponds in frequency to the synchronizing pulses, it may beused to develop a frequency stabilizing voltage when applied to thebalanced peak rectifier, just as in the case of Fig. 1.

The embodiment of our invention shown in Fig. 3 has a number ofadvantages. In the first place, the number of synchronizing pulses isreduced by the frequency step-up used in multivibrator IU. This reducesproportionally the radio-frequency power requirements for these pulses,thereby reducing the required transmitting capacity. In addition, theinterval between successive synchronizing pulses is increased over thatwhich would otherwise be required. This further reduces the possibilitythat noise and undesired random transients will trigger the system,thereby providing a more satisfactory image under adverse conditions ofnoise.

A further advantage of the circuit of Fig. 3 over other synchronizingcircuits resides in the fact that the pulses from multivibrator I0following each synchronizing pulse are of uniform frequency. Thisresults from the fact that under steady conditions the phase controlsystem exercises primary control over the multivibrator frequency andchanges that frequency until synchronization is achieved. If only thedirect synchronization through capacitor 9 were provided, thesynchronizing pulses would be effective only to trigger themultivibrator and subsequent oscillations would take place at the freerunning frequency of that unit until occurrence of the nextsynchronizing pulse. Hence correspondingly poor synchronization would beobtained.

It is possible to operate the system of Fig. 3 without the gearmultivibrator 60, but in this case the system is somewhat less stablethan that shown in the figure.

In the circuits of Figs. 1 and 3, tube 5| is connected in a cathodefollower circuit. This connection permits matching the impedance oftransformer 42 to the synchronizing pulse in ut, which is important toprovide optimum operation of the system and minimum distortion of thepulse wave applied to the balanced rectifier 35.

In both Figs. 1 and 3, the amplifier 26 is shown as shunted by anadjustable resistor 21 and a capacitor 28 in series. The purpose ofresistor 21 and capacitor .28 paralleling the feedback loop in Figs. 1and 3 is to provide a damping circuit to prevent hunting of the system.Capacitor 24, in parallel with this damping circuit further reduces theeffect of noise components and stabilizes operation of the system.Proper selection of these capacitors and adjustment of resistor 21provides a system that is highly stable in operation.

While we have shown and described our invention with reference toparticular embodiments thereof, it will be obvious to those skilled inthe art that changes and modifications may be made without departingfrom our invention in its broader aspects. For instance, the particularform of balanced rectifier 35 shown in the drawings may be replaced byother known circuits providing similar performance. In some cases it maybe desirable to omit the coupling amplifiers 26 and While our inventionhas been shown in its application to horizontal synchronizing circuitsof a television receiver, it obviously may also be applied to thevertical synchronizing circuits or to other forms of pulse apparatus. Weaim in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of theUnited States, is:

1. A synchronizing system comprising, in combination, a source ofperiodic synchronizing pulses, a square wave generator having afreerunning frequency adapted to be synchronized with said pulses, asawtooth wave generator adapted to be synchronized by said square wave,means for supplying said square Wave to synchronize said sawtoothgenerator, means comprising a phase detector responsive to both saidsynchronizing pulses and said sawtooth waves for developing aunidirectional control potential dependent upon their relative phase,means for altering said free-running frequency of said square wavegenerator in response to said potential in a sense tending to maintainsaid potential constant, and means for additionally synchronizing saidsquare wave generator directly from said synchronizing pulses.

2. A synchronizing system comprising, in combination, a source ofsynchronizing pulses of predetermined frequency, a multivibrator forgenerating control pulses and having a free-running frequency adapted tobe synchronized with said predetermined frequency, a generator ofsawtooth waves operating at substantially said predetermined frequency,means for synchronizing said sawtooth wave generator from said controlpulses, means comprising a phase detector responsive to the time phasebetween said synchronizing pulses and said sawtooth waves forcontrolling the free-running frequency of said multivibrator, means foradditionally synchronizing said multivibrator directly from saidsynchronizing pulses, and output means utilizing said control pulses.

3. In combination, a source of periodic voltage pulses, a multivibratorhaving a first operating condition and a second operating condition anda natural frequency of change between said operating conditions, saidnatural frequency being of value to cause said multivibrator to tend tooperate through a plurality of cycles for each of said pulses, meansresponsive to said pulses tending to cause said multivibrator to changefrom said first operating condition to said second operating conditionwhen said pulses take place, means to generate a sawtooth voltage wave:each time said multivibrator changes from said first operating conditionto said second operating condition, a phase detector, means forimpressing both said periodic voltage pulses and said sawtooth wave onsaid phase detector, means comprising said phase detector to generate aunidirectional voltage dependent on the voltage of said sawtooth wave atthe instant said pulses occur, and means responsive to said last voltageto alter the natural frequency of said multivibrator in a sense to causesaid sawtooth wave to occur in synchronism with said pulses.

4. In combination, a source of periodic voltage pulses, twomultivibrators each having a first operating condition and a secondoperating condition and a natural frequency of change between saidoperating conditions, the natural frequency of one of saidmultivibrators being approximately equal to an integral multiple of thefrequency of said pulses and the natural frequency of the other of saidmultivibrators being approximately equal to the frequency of saidpulses, means tending to cause said first multivibrator to change fromsaid first operating condition to said second operating condition whensaid periodic voltage pulses take place, means tending to cause saidsecond multivibrator to change from its first operating condition to itssecond operating condition when said first multivibrator changes fromits first operating condition to its second operating condition, meansto generate a sawtooth voltage wave each time said second multivibratorchanges from its first operating condition to its second operatingcondition, a phase detector, means for impressing both said periodicvoltage pulses and said sawtooth wave on said phase detector, meanscomprising said phase detector to generate a unidirectional voltagedependent on the voltage of said sawtooth wave at the instant saidpulses occur, and means responsive to said last voltage to alter thenatural frequency of said first multivibrator in such sense as to causesaid sawtooth wave to occur in synchronism with said pulses.

5. In a synchronizing system responsive to a synchronizing pulse wave ofpredetermined frequency, the combination of a square wave generatoradapted to be synchronized by said pulse wave and having a higher normaloperating frequency equal to an integral multiple of said predeterminedfrequency, means for producing a periodic voltage wave normallyrecurring at said predetermined frequency and synchronized rigidly withsaid square wave, a phase detector, means for impressing both saidsynchronizing pulse wave and said periodic voltage wave on saiddetector, means comprising said phase detector for producing controlpotentials dependent upon the time phase between said synchronizingpulse wave and said periodic voltage wave, means utilizing saidpotentials for varying the normal operating frequency of said squarewave generator in such sense as to maintain it in synchronism with saidperiodic voltage wave, means for additionally impressing saidsynchronizing pulse wave on said square wave generator directly toassist in maintaining said synchronism, and utilization means responsiveto said control pulse wave.

HUGH B. FLEMING. GEORGE M. BROWN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,141,343 Campbell Dec. 2'7, 19382,277,000 Bingley Mar. 17, 1942

